A plasma display panel is a display apparatus which contains a plurality of discharge cells, and is constructed to display an image by applying a voltage across electrodes discharge cells thereby causing the desired discharge cell to emit light. A panel unit, which is the main part of the plasma display panel, is fabricated by bonding two glass base plates together in such a manner as to sandwich a plurality of discharge cells between them.
In a plasma display panel, the discharge cells which are caused to emit light for image formation generate heat and each thus constitutes a source of heat, which causes the temperature of the plasma display panel as a whole to rise. The heat generated in the discharge cells is transferred to the glass forming the base plates, but heat conduction in directions parallel to the panel face is difficult because of the properties of the glass base plate material.
In addition, the temperature of a discharge cell which has been activated for light emission rises markedly, while the temperature of a discharge cell which has not been activated does not rise as much. Because of this, the panel face temperature of the plasma display panel rises locally in the areas where an image is being generated, accelerating thermal deterioration of affected discharge cells, unless some heat sinking measures are taken.
Further, since the temperature difference between activated and nonactivated discharge cells can be high, and, in fact, the temperature difference between discharge cells generating white light and those generating darker colors also can be high, a stress is applied to the panel unit, causing the conventional plasma display panel to be prone to cracks and breakage.
When the voltage to be applied to the electrodes of discharge cells is increased, the brightness of the discharge cells increases but the amount of heat generation in such cells also increases. Thus, those cells having large voltages for activation become more susceptible to thermal deterioration and tend to exacerbate the breakage problem of the panel unit of the plasma display panel.
The use of graphite films or sheets as thermal interface materials for plasma display panels has been suggested by, for example, Morita, Ichiyanagi, Ikeda, Nishiki, Inoue, Komyoji and Kawashima in U.S. Pat. No. 5,831,374. In addition, the heat spreading capabilities of sheets of compressed particles of exfoliated graphite has also been recognized. Indeed, such materials are commercially available from Advanced Energy Technology Inc. of Lakewood, Ohio as its eGraf® 700 class of materials.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional such as thermal and electrical conductivity.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal conductivity due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from high compression, making it especially useful in heat spreading applications. Sheet material thus produced has excellent flexibility, good strength and a high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cc to about 2.0 g/cc.
The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon compression of the sheet material to increase orientation. In compressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.
One drawback to the use of graphite sheets as heat spreaders for plasma display panels lies in the plasma display panel manufacturing process. More specifically, plasma display panels are produced in very high volumes, and the process for applying a graphite heat spreader to the plasma display panel need be such that a bottleneck in the manufacturing process is not created. Moreover, a means of adhering the graphite spreader to the panel is needed to avoid having the graphite spreader fall off during the manufacturing process and to ensure good thermal contact between the graphite spreader and the plasma display panel without the requirement of high pressure application of the spreader; however, the attachment method must not have a significant deleterious impact on the thermal performance of the heat spreader.
One method for attaching a graphite heat spreader to a plasma display panel is by use of an adhesive applied to the graphite. U.S. Pat. No. 6,245,400 to Tzeng, Getz and Weber describes a method for producing a release-lined pressure sensitive adhesive flexible graphite sheet article, wherein the release liner is easily removed from the graphite sheet without delaminating the graphite. Graphite sheet has a relatively low cohesive strength and removing the release liner without delaminating the graphite is a significant challenge. A key component of the Tzeng et al. patent is the use of a primer coating applied to the graphite sheet prior to applying the pressure sensitive adhesive. The disadvantage of this approach is the need for an additional coating step, which increases manufacturing complexity and cost.
Thus, a method for producing a release lined pressure sensitive adhesive flexible graphite sheet without the use of a primer coating is needed for use as a heat spreader for plasma display panels. Furthermore, this method should allow for the achievement of very high speed of release of the release liner from the adhesive-coated graphite sheet, without delamination of the graphite, and yet not create an undesirably high reduction of the thermal properties of the graphite heat spreader.